A method for improving Milky Way exposures in light pollution
Light pollution is a rapidly growing problem. Most of the population in the United States cannot see the Milky Way. When I look up at the sky from Venice, California, I can see about 20 stars. The rest of the galaxy is hidden by an orange haze of light pollution. This guide talks about a method called ETTR or “Exposure To The Right” that I have used successfully to actually be able to photograph the Milky Way in these terrible conditions.
But why should you care about seeing the Milky Way?
Because the Milky Way is the very place that we were all born. It is the cradle of our star, the Sun, and our planet, the Earth and everything on it. It’s there, visible to all in a dark enough sky, it’s long spiral arms stretching across the sky, pink nebula dotted against dark dust clouds and it’s bright dense center housing a massive black hole. It is where we are in the universe.
Someday, images of the Milky Way may be only historical photos of yesteryear, a once-was-a-possibility of the distant past. Light pollution is growing at an alarming rate and someday, if this trend continues, it will be impossible to see our own galaxy from the surface of the Earth.
I live in Los Angeles, California. If there is a place that’s well known for its clear, dark starry skies, LA isn’t it, not by a long shot. Every past photograph of the Milky Way that I have taken was made far away from the LA city lights, in distant places like Death Valley National Park and Joshua Tree National Park, hundred of miles away from the city lights of LA.
Even so, I made it a mission to try and find a nearby place that I could go to observe and photograph the milky way. The photo of the park bench here, believe it or not, was taken very near Los Angeles. The Milky Way was just barely visible against a hazy sky, lit profusely by the orange-yellow glow of the sodium vapor street lamps of the city. If you didn’t know what you were looking for, you wouldn’t know that you were seeing the center of our galaxy.
My girlfriend and I recently drove a short distance from Venice, California to Rancho Palos Verdes. In Palos Verdes, there’s a park called Del Cerro Park that’s propped on the top of a high cliff wall, looking out toward Catalina Island. It’s certainly no astronomer’s paradise. It’s just adjacent to the Port of Long Beach, a major light pollution source and there are still a plethora of street lamps scattered all over the Palos Verdes neighborhoods. It’s also only 25 miles from downtown Los Angeles and 15 miles from downtown Long Beach. And to make it even more difficult, it’s just by the seaside where a nightly marine layer forms. This high concentration of water vapor picks much of the light pollution, turning the sky a dull muddy orange color.
The only saving grace of Del Cerro Park was the fact that it looks out to the south over the Pacific Ocean. The center of our galaxy just happens to be in the southern half sky. When we arrived, it was just barely possible to see some faint silvery glow against the orange light pollution haze.[map width=”100%” height=”250″ zoom=”8″ type=”TERRAIN”] [marker address=”Del Cerro Park, Rancho Palos Verdes, CA”]Del Cerro Park, Rancho Palos Verdes, CA[/marker] [/map]
In this light polluted environment, my typical milky way exposure of 30 seconds at f/2.8 and ISO 6400 looked like it wouldn’t quite work (I was using a 14mm f/2.8 lens on a Canon EOS 6D). There was just too much light pollution and the resulting images seemed totally washed out. So I reduced my exposure value a whole 3 stops to 15 seconds at f/2.8 and ISO 1600.
In the end this change was a small mistake. I should have stuck with the washed out image. This seems counter-intuitive and even looking at the unprocessed results, it make even less intuitive sense. The higher exposure image at the higher ISO picked up way more light pollution than the lower exposure image at lower ISO. The result seemed washed out and overexposed. But I maintain that my brighter, standard exposure is the better picture in the end, despite the fact that it looks way overexposed. This is actually a method of exposure called “Expose To The Right“, often initialized as ETTR. Let me show you why it give better results.
Expose To The Right (ETTR)
Expose To The Right, or ETTR as it’s usually called, is a method of exposure optimization that utilizes overexposure in order to get the highest dynamic range by collecting the maximum amount of light data. It’s called Expose To The Right in reference to how the histogram of luminosity data should look for an image shot with this method. The majority of the image data should be pushed to the far right of the histogram without pushing the data over the edge (clipping). This ensures that most of the data recorded is in the “bright” section of the graph.
Here are two, similar, unprocessed RAW images of the Milky Way taken at Del Cerro Park with two different exposures notice how the histograms differ. The first image shows the data shifted all the way to the right, but not so far as to clip the data at the edge of the graph. The second image shows a relatively neutral looking exposure, perhaps just a tad underexposed.
The first image is a full 3 stops brighter and looks way overexposed and the second image looks a little bit more like it did in real life. In the second image, you can just barely make out a hazy bright spot in the center of the frame. That’s the Milky Way galactic center. Either way, both images are tremendously awash with light pollution from the nearby cities and are rather flat from the dull orange light pollution and the fact they they are unprocessed RAW images, straight from the camera.
Now let’s see what we can do with some post-processing. The very first thing I did was use Adobe Lightroom’s exposure adjustment slider to reduce the exposure of the first image by 3 stops to match the second exposure. Then, I adjusted both images identically: I changed the white balance to about 3200°K to compensate for the orange light pollution and I increased contrast as much as possible without blowing out the highlights or darks too much. I wanted the sky to look relatively dark to keep the night time feel but wanted to boost the brightness of the Milky Way as much as possible. It was also necessary to correct for some lens vignetting and the light gradient that the nearby Port of Long Beach created with its overwhelming light pollution. Here are the results on both images:
Wow! both images actually have a fair amount of detail of the Milky Way. This is a true testament to the capabilities of a modern digital camera like the Canon EOS 6D.
And as I expected, the heavy amount of contrast increase necessary in post processing made the influences of light pollution more pronounced. You can see the very bright orange glow on the left side of the frame from the Port of Long Beach. On the right is light pollution from street lamps and houses in the town of Rancho Palos Verdes.
Both of these images were processed exactly the same, except the first overexposed image was “normalized” by reducing exposure in post-processing by exactly -3.0 EV to match the exposure value of the second image. Interestingly, the initially overexposed image made at higher ISO actually shows cleaner details and shows less noise than the image taken at lower ISO. Here’s a close up of the details on both images:
The first image shows less luminance noise and less color noise and a little cleaner, smoother overall detail than the second image. So what’s going on here? Shouldn’t the image taken at ISO 6400 show more noise than the image taken at ISO 1600? If the exposures were the same brightness, yes. But these images were not taken with the same exposure. The first image started with twice as much shutter open time (30 seconds versus 15 seconds) and two stops more gain on the sensor (ISO 6400 vs ISO 1600). The result is that the first image has significantly more light data. This makes the final signal-to-noise ratio of the processed images higher on the first image. The higher the signal to noise ratio, the less noisy the resulting photo. So in this case it was actually better to overexpose and compensate later in post processing, despite the initial unprocessed images appearing unusable. This is a great example of Expose To The Right exposure optimization.
The results are here are a simple lesson in this type of photography: The more signal, the better. This is why my How-To Photograph the Milky Way article stresses the importance of a fast lens with as low an f-stop rating as possible (my favorite is still the Rokinon 24mm f/1.4). The larger aperture on these lenses collects significantly more light, allowing for a higher signal exposure.
Luckily, the dynamic range of light on a dark night (even in light pollution) is significantly lower than in a daytime photograph under the bright sun. This gives us a unique opportunity to be able to push ISO sensitivities to relatively high levels in order to detect the weakest details of light in the night sky and not worry about clipping or blowing out the highlights of the image. This makes astrophotography an ideal application for ETTR in order to boost signal, even if we are achieving the ETTR result by using a much higher ISO. Note, however, that this does not apply to the extended or expanded ISO range on your camera (which achieves the higher sensitivity artificially by underexposing at a lower ISO and then boosting the exposure in processing, the exact opposite of ETTR).
So in general, avoid underexposing your astrophotos at all costs. It’s remarkable what you can recover from what seems like an overexposed image. Here are a couple more images from that night that I hope you will enjoy. (Hover over the images to see their original RAW unprocessed versions):[himage]
Above is Diana and me on one of the lookout points at Del Cerro Park. On the horizon, you can see clouds forming along the distant shoreline of Catalina Island, a couple ships off the coast and, of course, the orange glow on the left from the Port of Long Beach.[himage]
And this is us sitting on the fence on the edge of Del Cerro Park. Below, you can see the lights of Rancho Palos Verdes and the edge of San Pedro. On the left side of the frame in the sky you can barely see a visible pass by the Cosmos 1484, a portion of a 1983 launched Russian rocket still in orbit around the Earth (Time of pass: 3:24 am, March 19, 2013). Shortly after this photograph, the marine layer started to form and the condensation in the air made the conditions markedly worse for photography than earlier in the night. By 4 am, we finally decided to call it a night.[himage]
The results of the night were an interesting lesson in light pollution and the capabilities of a modern digital camera. While I’m surprised and quite happy at the results, I know that without the terrible Los Angeles light pollution, our view that night would have been that much more spectacular. I invite you to learn more about light pollution from the International Dark Sky Association.
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